1. Field of the Invention
The invention relates to oxide ceramic fiber composite materials as used, e.g., according to the invention in energy conversion installations.
2. Discussion of Background Information
In the interest of energy outputs to be substantially increased in the future, greatly increased efforts are to be observed to provide ceramic materials for energy production installations at temperatures ≧1400° C. Concurring assessments thereby assume that for load-bearing components under such extreme conditions:                It will be possible to realize both an adequate damage tolerance (i.e., thermal shock stability and high room temperature fracture toughness of the actually brittle ceramics) and the required creep stability of the components only by means of a fiber-composite design, and        The problem of thermodynamic high-temperature stability in oxidizing atmospheres can be permanently mastered only with oxide ceramics that are oxidation-stable per se.        
High-performance brake disks have to meet similarly differentiated requirements regarding damage tolerance, temperature stability and corrosion stability. Currently the mechanical and thermophysical requirements are best met by composites of carbon long fibers with reaction bonded SiC matrix. However, the corrosion resistance of these composites is limited due to corrosion progressing along the non-oxide fibers into the interior of the disks. It was observed that, although the use of carbon short fibers improves corrosion stability, it limits the mechanical property level (R. Gadow, p. 15–29 in: Ceram. Eng. Sci. Proc. Vol. 21/3, The Am. Ceram. Soc., Westerville/OH, 2000). Purely oxidic composites could therefore provide an advantageous alternative material here too.
In view of the complex mechanical requirements, it is evident with the above assessments for all such uses that the mechanical properties of the fiber/matrix bond are of decisive importance in realizing such oxidic composite materials. However, it is also evident why it has hitherto been impossible to develop a concept that meets the demands for thermal shock stability and for creep resistance equally:                The requirement for transferring the creep resistance of a (preferably multi-dimensional) fiber structure to the component as a whole, at least in view of a long-term use of ≧10,000 hours, leads to the obvious requirement for a strong fiber/matrix bond (I. W. Donald et al., J. Mater. Sci. 11(1976) 5, 949–972); A.G. Evans et al., p. 929–956 in Creep and Fracture of Engineering Materials, London, 1987). Although one could hope through a multi-dimensional fiber design to achieve a certain macroscopic form stability of the parts even without a strong fiber/matrix bond solely through a high creep resistance of the fiber arrangement, very complex stresses (also with regard to an erosion stress) exist in turbines under the conditions of flowing atmospheres and high pressure gradients, which composite materials with generally weak interfaces can hardly hold out for longer periods at temperatures about 1400°C.        In contrast, the requirement for a ceramic that is repeatedly thermal shock-resistant seems currently to be realizable exclusively by a composite design that realizes a consumption of (brittle) fracture energy (energy dissipation) after strain and partial fracture of fibers by means of a considerable shearing deformation along the fiber/matrix interfaces (crack deflection, “pull-out” or “debonding” effects, rendered possible by weak fiber/matrix bond).        
In implementing this realization a number of proposals have been made which all without exception contain formation of weak fiber/matrix interfaces. This is carried out in part by fiber coating and in part by a corresponding design of the matrix.
Pejryd et al. (EP 639 165 A1, U.S. Pat. No. 5,567,518) thus describe “a ceramic composite particularly for use at temperatures above 1400° C.” with a structure of oxide fiber/oxide matrix and a material selection determined by claim and, if necessary, larger coating thickness of the fibers >7 μm such that expressly weak interfaces (“weak bond liable to debonding”) are achieved. As in most of the proposals known from the literature, however, the real usefulness for the declared application is not disclosed here either: the “proof” of an energy consumption during fracture (in the form of non-linear stress-strain effects) occurs in the examples only through tests at room temperature, which does not allow any conclusions to be drawn regarding real thermal shock behavior or certainly regarding creep stability with the uses above 1400° C. given as the object.
Saruhan-Brings et al. (EP 890 559 A1) describe a “Process for coating oxidic fiber materials for producing failure-tolerant, high-temperature resistant, oxidation-resistant composite materials.” As an example a La aluminate coating on polycrystalline Al2O3 fiber is bonded to a mullite matrix. No high-temperature tests or their results are disclosed. The same defect is shown by the patent of Lange et al. (U.S. Pat. No. 5,856,252) where within the scope of “damage-tolerant ceramic matrix composites by a precursor infiltration” a claim describes an “all oxide ceramic composite” of uncoated Al2O3 fibers and porous mullite matrix without disclosing information on the high-temperature behavior; the main content of U.S. Pat. No. 5,856,252 is the description of oxidic composites with damage-tolerant behavior achieved through “delaminations . . . with extensive regions of cracking normal to the rupture plane.”
A similar approach is found with Dariol et al. in a “processing method for an interphase material, material obtained, treatment process of a ceramic reinforcement fiber with this material and thermostructural material including such fibers” (FR 27 78 655 A1), where the formation of a microporous fiber/matrix interface area occurs through the use of a pore-forming additive (e.g., carbon that is at least partially oxidized). Lundberg et al. (EP 946 458 A1) have obtained a similar microstructural result. They produced microporous fiber/matrix interfaces for oxide fiber/oxide matrix composites, intended specifically for use in oxidizing atmospheres >1400° C. by immersing the fibers in powder slurry containing carbon and ZrO2.
The proposed solutions thereby overlook the fact that the creep rates particularly of the oxides in question between 1300° C. and 1700° C. increase by approx. 4 orders of magnitude (many non-oxide ceramics are more creep resistant, but are thereby oxidation-susceptible) so that an adequate mechanical long-term stability (≧10,000 h) will be completely impossible to realize with polycrystalline fibers in polycrystalline matrices.
The present invention is therefore based exclusively on composites of oxide ceramic materials with monocrystalline fibers at least in some areas whose average coherence lengths of the monocrystalline areas are at least 150 μm, preferably ≧400 μm, in particular preferably ≧1 mm. Surprisingly, in the above-referenced publications only in EP 639 165 A1 is there an exemplary embodiment that meets this requirement in that thick monocrystalline sapphire fibers (Saphikon, USA; fiber thickness ≧100 μm) are coated with ZrO2 and embedded in an Al2O3 matrix. However, EP 639 165 also lacks any information on the actual behavior under temperature stress relevant as a target value. Also with use or in-situ production of monocrystalline fibers, the concept of weak interfaces is adhered to with the consequence that in the range of low temperatures the fibers tend to reduce rather than improve strengths (A. A. Kolchin et al., Composite Sci. Technol. 61 (2001)8, 1079–1082).
The previously known proposed solutions therefore have in common that even in the case of a target definition for use at temperatures ≧1400° C., the development of properties is oriented exclusively to the thermal shock stress relevant only at lower temperatures and that no disclosure is made of the material properties even here. All known developments have as their goal microstructural construction according to “damage-tolerant” behavior which is realized by weak fiber/matrix interfaces. Incomprehensibly, the achievement of this object is not proven in any case by corresponding thermal shock tests in technically relevant temperature ranges; instead all the listed exemplary embodiments are limited to simply room temperature fracture tests.
When this “weak” fiber/matrix bond is produced by bond-weakening fiber coatings with suitable materials or through artificially produced micropores, it is still overlooked that such micropores in the target use temperature range become unstable, at least in long-term use, and that the bond-weakening effect of a fiber coating is also quite different at higher temperatures, such as, e.g., at 1400° C., than at the room temperature range exclusively tested.
Of course “weak” fiber/matrix interfaces also cannot meet the dominant requirement for creep stability particularly at ≧1400° C.
Therefore, the known approaches for oxide ceramic fiber composite materials for use at temperatures ≧1400° C. do not provide a practical solution or even a hint for the thermal shock stresses associated with heating up/cooling down, or for the serious problem at such high temperatures of creep deformation.